The friction composition for railroad brake shoes usually contains four functional groups of ingredients, namely bonding materials, organic and carbon based fillers, inorganic fillers and reinforcing fibers including synthetic and natural organic fibers. The bonding materials provide a curable matrix to hold a uniform dispersion of the fillers and reinforcing fibers and usually comprise both a vulcanizable natural or synthetic rubber and a thermosetting resin of natural or synthetic source. Typically the rubber component has been butyl rubber, a styrene butadiene copolymer rubber, an acrylonitrile rubber, a chlorinated butyl rubber, or mixtures thereof, along with a curing agent such as sulfur or 2-mercaptobenzothiazole, tetramethylthiuram disulfide and may also include minor amounts of stiffening agents such as carbon black, magnesium oxide and zinc oxide. Normally the thermosetting resin has been a phenol-aldehyde resin or cashew nut shell oil partially polymerized with an aldehyde, and mixtures thereof. If the resin component is not self-curing, an independent curing agent such as hexamethylene-tetramine is included in the bonding matrix.
Suitable organic and carbon based fillers have included petroleum coke, coal, graphite (both natural and synthetic), neoprene scrap and fully cured cashew nut resin. The purpose of such organic and carbon based fillers is to provide bulk to the final composition and to control friction at elevated operating (braking) temperatures.
Inorganic fillers function to largely supply the frictional coefficient of the final composition and to absorb frictional heat during braking and typically have included iron grit, iron powder, red iron oxide, sand, aluminum silicate (kyanite), barytes, rottenstone, clay, aluminum trihydrate, magnesium oxide and zinc oxide.
The reinforcing fibers serve to provide tensile strength and shear resistance in the final composition. Historically, asbestos was favored as a natural reinforcing fiber. Other natural fiber sources are wollastonite (calcium silicate) and mica. Manufactured fibers include glass fibers, steel wool, alumna-silica ceramic fibers and most recently favored aramid polymer fiber and acrylic fibers.
Examples of prior railroad brake shoe production and the ingredients and methods therefore are disclosed in U.S. Pat. Nos. 4,169,680, 4,219,452, 4,313,869 and 4,466,513 the details of which are included herein by reference.
Heretofore it has been necessary to dilute the bonding materials with a volatile solvent such as toluene so as to provide sufficient fluidity to enable the additional ingredients to be wetted and dispersed therein and to permit introduction of fragile reinforcing fibers without destroying or damaging the structure thereof. Subsequently, the solvent was driven out of the composition by vacuum heating. The need for such a solvent has been a disadvantage for a number of reasons including the cost of the solvent and equipment to recover and recycle same. Moreover, it has been nearly impossible to prevent loss of some solvent with attendant environmental and employee health concerns.